1. FIELD OF THE INVENTION
This invention relates to helical, rotary screw compressors, and more particularly, to a simplified system of liquid injection to control and limit the discharge temperature of the refrigerant working fluid and the lubricating oil carried thereby.
2. DESCRIPTION OF THE PRIOR ART
In general, compressors are pumps that are used to raise gases or refrigerant from one pressure level to a higher pressure level. In the process, the vapor or gas is superheated by the work of compression. Through thermodynamic relationships, operating temperatures can be predicted by applying isotropic or polytropic compression processes. With all types of compressors, the higher the compression ratio EQU [compression ratio=(discharge pressure/section pressure)]
the higher the discharge temperature that will be reached.
It is desirable to control and limit discharge temperatures so that dangerous levels are not reached that may injure components and lubricants and shorten their useful life. In the past, many methods have been employed to inject fluids into gas streams for the purpose of cooling or limiting compressor discharge temperature. In air compressors, water mist has been sprayed into the compression area, which vaporizes during compression to thereby limit temperatures. In other compressors, oil injection has been used to accomplish lower discharge temperature.
Attempts have also been made to inject liquid refrigerant into the refrigerant vapor or working fluid as it is being compressed. This has been accomplished by injecting liquid refigerant or refrigerant rich oil into the suction side of the compressor where the refrigerant evaporates and reduces the net inlet suction volume of the compressor, decreasing the capacity of the compressor. Attempts have further been made to add liquid to the gas discharge from the compressor.
In the conventional systems employing axial screw compressors, the need for oil cooling limits the discharge temperatures that the system can tolerate. In systems with water cooled oil coolers, the range of operation is usually established by water temperatures available. In the case of minimizing the use of water or in air cooled systems due to the ambient temperature of the air, there is a problem in maintaining tolerable discharge temperatures. One way of maintaining the discharge temperatures is through the use of liquid injection along with some oil injection. In such a case, location of the port for the liquid injection is critical in that, if the liquid injection port is on the inlet or suction side of the machine, the effect of the liquid being injected greatly affects the volumetric efficiency of the compressor due to the fact that the liquid will expand, flash off as it hits the low pressure environment. There is also an effect on the horsepower requirements of the machine, because the expanded liquid then is in gas form and goes through a pressure range change and exits at the machine discharge pressure.
If the point of liquid injection occurs as the discharge side of the machine or after the gas has actually left the compressor discharge area, not only is the pressure condition at the system highest and thus there is an inherent requirement for an external pump to pressurize the liquid to be injected above the maximum compressor pressure of the system, but the added unit constitutes an extra component which adds to the cost of the unit, and affects the reliability of the system. Most importantly, where compressive systems are designed hermetically, there is a rather confined distance from the point of compressor gas discharge from the screw compressor itself to where the same gas contacts and envelops the motor winding of the hermetic electric motor and the space between the two does not provide sufficient time or room for the liquid injected at this point to properly expand and cool the discharge gas prior to entering the motor compartment.
Conventionally, lubricating oil has been injected into the working chamber, that is, the space occupied by the intermeshed screws in a helical rotary screw compressor, for the dual purposes of lubricating the intermeshed screws and to provide the necessary seals between the rotating screws and the stationary housing. Further, since the load on the compressor varies at times between relatively large limits, the capacity of helical rotary screw compressors has been modified by incorporating a capacity control slide valve within the rotor housing and slidable parallel to the axis of the screw. Axial movement of the valve is programmed by a solid state, temperature initiated hydraulic actuated control arrangement. The slide valve shifts longitudinally between limits with the slide valve in closed position and against a valve stop when the compressor is fully loaded, in which case all the gas flows through the rotor housing from the intake to the discharge side of the screw compressor. Unloading is achieved by moving the valve away from the valve stop to create an opening within the rotor housing through which the suction gas can return to the inlet port area before compression of the same. Thus, in principle, enlarging the opening in the rotor housing effectively reduces compressor displacement. One mode of insuring that lubricating oil is injected into the working chamber and between the intermeshed screws has been to provide an axial passage in the mechanism connecting the slide valve to a reciprocating fluid motor and creating a closed chamber at the discharge side of the slide valve with one or more radial ports opening up into the working chamber downtream of the contact area between the end of the valve and the stationary valve stop. In this case, as the slide valve opens to reduce the capacity, oil injection occurs within the working chamber closer to the discharge side of the compressor.
It is, therefore, an object of the present invention to eliminate the necessity for a separate pump in liquid injection cooling of a screw compressor and to effect liquid injection cooling of a screw compressor without materially affecting the volumetric capacity of the compressor or increasing the horsepower required.